Corner reflectors (also known as "corner cubes") are well known reflective devices. A light ray incident upon a corner reflector undergoes total internal reflection in each of three separate reflections at the three perpendicularly opposed facets which form the corner, with the net result that the light is retro-reflected from the corner reflector in a direction opposite to the direction of the incident ray.
Miniaturized transparent groupings of corner reflectors, each of which reflectors exhibit the abovedescribed phenomenon of total internal reflection, are commonly found in reflective sheeting materials such as 3M Diamond Grade.TM. reflective sheeting. A group of one or more corner reflectors can be made to function as an image "pixel" by switching the total internal reflection phenomenon on or off. An array of such pixels can be assembled to construct a display device capable of displaying text or images.
FIGS. 1A and 1B depict, in cross-section, a grouping 10 of retro-reflective elements, namely corner reflectors. Only two facets of each corner reflector 12A, 12B, 12C, etc. are visible in such a sectional view, but persons skilled in the art will understand that each corner reflector has three perpendicularly opposed facets. Corner reflector grouping 10 may be a sheet of corner cube film such as that found in 3M Diamond Grade.TM. reflective sheet film material.
It is well known that light travels at different speeds in different materials. The change of speed results in refraction. The relative refractive index between two materials is given by the speed of an incident light ray divided by the speed of the refracted ray. If the relative refractive index is less than one, then light will be refracted towards the surface, eg light emerging from a glass block into air. At a particular angle of incidence "i", the refraction angle "r" becomes 90.degree. as the light runs along the block's surface. The critical angle "i" can be calculated, as sin i=relative refractive index. If "i" is made even larger, then all of the light is reflected back inside the glass block and none escapes from the block. This is called total internal reflection. Because refraction only occurs when light changes speed, it is perhaps not surprising that the incident radiation emerges slightly before being totally internally reflected, and hence a slight penetration (roughly one micron) of the interface, called "evanescent wave penetration" occurs. By interfering with (i.e. scattering and/or absorbing) the evanescent wave one may prevent total internal reflection.
In FIG. 1A, grouping 10 is "on", such that incident light ray 14 is retro-reflected by corner reflector 12D due to the phenomenon of total internal reflection. Corner reflector grouping 10 thus constitutes a single "pixel" which can be made to appear white when "on", due to the high reflectivity exhibited by the corner reflectors. In FIG. 1B, corner reflector grouping 10 is "off", such that incident light ray 16 is not reflected by corner reflector 12D due to prevention of the phenomenon of total internal reflection. When in the "off" state, grouping 10 can easily be made to appear black, due to the low reflectivity exhibited by the corner reflectors in the off state. An array of such "pixels", each comprising a separate grouping of corner reflectors can accordingly be assembled to form a black on white display capable of displaying text or images.
One way of switching the total internal reflection capability of corner reflector grouping 10 on or off is to mount a sheet of elastomeric film material 18 adjacent the rear surface of corner reflector grouping 10, as seen in FIGS. 1A and 1B. In FIG. 1A, a small gap 20 is left between the adjacent faces of the sheet film materials comprising corner reflector grouping 10 and elastomeric sheet 18. With gap 20 present, elastomeric sheet 18 has no effect on corner reflector grouping 10. This is because gap 20 is much larger than one micron and therefore does not interfere with the evanescent wave and hence does not prevent the total internal reflection capability of corner reflector grouping 10. Thus, the "pixel" formed by corner reflector grouping 10 is "on" if gap 20 is present.
However, in FIG. 1B, control means 19 has been activated to move elastomeric sheet 18 in the direction of arrow 21 such that the adjacent faces of corner reflector grouping 10 and elastomeric sheet 18 are in "optical contact" with one another. Optical contact between elastomeric sheet 18 and corner reflector grouping 10 brings elastomeric sheet 18 substantially closer than one micron to corner reflector grouping 10, thereby scattering and/or absorbing the evanescent wave adjacent corner reflector grouping 10, thus preventing the capability of corner reflector grouping 10 to totally internally reflect incident light ray 16. The "pixel" formed by corner reflector grouping 10 is accordingly "off" if the adjacent faces of corner reflector grouping 10 and elastomeric sheet 18 are in optical contact with one another, with no gap between them.
The present invention pertains to a suitable form of control means 19 capable of displacing elastomeric sheet 18 through the small displacements required to either form gap 20 or to achieve optical contact between elastomeric sheet 18 and corner reflector grouping 10.